The presence of volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), pesticides, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs) in subsurface soils and groundwater is a well-documented and extensive problem in industrialized and industrializing countries. Notable among these are the volatile organic compounds or VOCs which include any at least slightly water soluble chemical compound of carbon, with a Henry's Law Constant greater than 10.sup.-7 atm m.sup.3/mole, which is toxic or carcinogenic, is capable of moving through the soil under the influence of gravity and serving as a source of water contamination by dissolution into water passing through the contaminated soil due to its solubility, including, but not limited to, chlorinated solvents such as trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), 1,1-dichloroethane, 1,1-dichloroethene, carbon tetrachloride, benzene, chloroform, chlorobenzenes, and other compounds such as ethylene dibromide, and methyl tertiary butyl ether.
In many cases discharge of VOCs and other contaminants into the soil leads to contamination of aquifers resulting in potential public health impacts and degradation of groundwater resources for future use. Treatment and remediation of soils contaminated with VOCs and other organic contaminants have been expensive, require considerable time, and in many cases are incomplete or unsuccessful. Treatment and remediation of compounds that are either partially or completely immiscible with water (i.e., Non Aqueous Phase Liquids or NAPLs) have been particularly difficult. Also treatment of highly soluble but biologically stable organic contaminants such as MTBE and 1,4-dioxane are also quite difficult with conventional remediation technologies. This is particularly true if these compounds are not significantly naturally degraded, either chemically or biologically, in soil environments. NAPLs present in the subsurface can be toxic to humans and other organisms and can slowly release dissolved aqueous or gas phase volatile organic compounds to the groundwater resulting in long-term (i.e., decades or longer) sources of chemical contamination of the subsurface. In many cases subsurface groundwater contaminant plumes may extend hundreds to thousands of feet from the source of the chemicals resulting in extensive contamination of the subsurface. These chemicals may then be transported into drinking water sources, lakes, rivers, and even basements of homes through volatilization from groundwater.
The U.S. Environmental Protection Agency (USEPA) has established maximum concentration limits for various hazardous compounds. Very low and stringent drinking water limits have been placed on many halogenated organic compounds. For example, the maximum concentration limits for solvents such as trichloroethylene, tetrachloroethylene, and carbon tetrachloride have been established at 5 .mu.g/L, while the maximum concentration limits for chlorobenzenes, polychlorinated biphenyls (PCBs), and ethylene dibromide have been established by the USEPA at 100 .mu.g/L, 0.5 .mu./L, and 0.05 .mu.g/L, respectively. Meeting these cleanup criteria is difficult, time consuming, costly, and often virtually impossible using existing technologies.
Many methods exist for the remediation of soil, groundwater and wastewater to meet the clean-up standards. Examples include dig-and-haul, pump-and-treat, biodegradation, sparging, and vapor extraction. However, meeting stringent clean-up standards is often costly, time-consuming, and often ineffective for many compounds that are recalcitrant—i.e. not responsive to such treatment.
Chemical oxidation, either applied in situ or ex situ of the subsurface or waste stream, is an approach to treat contaminants with strong oxidizing chemicals, with the ultimate goal of complete mineralization, or conversion to carbon dioxide and water. Examples of oxidants that have been utilized for this purpose include Fenton's chemistry (activated hydrogen peroxide), permanganate and ozone. Persulfates, and in particular sodium persulfate, have more recently been suggested for use in environmental remediation through chemical oxidation.
One key aspect to the ability of an oxidizer to function is its ability to permeate through the subsurface, interacting with target compounds throughout the entire zone of contamination. Oxidizing species, such as peroxide, ozone and hydroxyl radicals have relatively short lifetimes within the subsurface. Persulfate radicals survive for greater periods. However there is a desire to have even longer lived active species available for organic species decomposition in order to increase the zone of reaction, without resorting to multiple injection points throughout the contamination area.